Definition of keys and values for Preferences.jl

[kimikage]

Preferences.jl is now used by several major packages.
As for the (weak) dependence of CheckedArithmeticCore on Preferences.jl, that is debatable.
However, I believe that the specifications of keys and values should be defined in CheckedArithmeticCore so that downstream packages can use Preferences.jl in a consistent manner.

[kimikage]

At the very least, I think that replacing all the arithmetic operations would have too great an impact.
In other words, the preferences should be interpreted on a package (module) basis.

Of course, we could add a drastic preference option for fundamentalists.

[kimikage]

A decision is needed on #12 before any work on this.